FIG. 3 illustrates a conventional V/I conversion circuit employing an IC. This circuit converts an input voltage V1 to an electric current Iout. In FIG. 3, Integrated Circuit (IC) 3 includes current sources I1, I2 and I3 and transistors Q1, Q2 and Q3. V2 is the supply voltage. A microprocessor is widely used to operate a device which employs the conventional V/I conversion circuit. When the microprocessor is used, generally, the output of the microprocessor is converted to a DC voltage using a D/A converter which is used as input voltage V1.
The input voltage V1 applied to the input terminal is compared with the voltage of node 1 including an emitter of Q1 and a base of Q2 using a differential amplifier circuit comprising Q2 and Q3. Then a voltage equivalent to the input voltage V1 is output to the base of Q1. Node 1 is connected to an external resistance R1 through a terminal. If R1 is provided as an internal resistance in the IC, the external resistance R1 is not needed. However, the internal resistance of the IC usually has a resistance value which varies widely causing a wide dispersion in output current Iout. The external resistance R1 is used in a V/I conversion circuit which requires accuracy.
The voltage of node 1 is maintained equivalent to the input voltage V1 using a negative-feedback-circuit comprising Q1 and Q2. As a result, the emitter current of Q1 becomes nearly equal to a value of divided voltage of node 1 by R1. When the current amplification factor of Q1 is high enough, the collector current is nearly equal to the emitter current, and the collector current is the output current Iout of the V/I conversion circuit. Since Iout equals V1/R1, Iout can be adjusted from outside of the IC by changing the input voltage V1.